There are several well known ways of coupling transducer piezoelements to the transmit and receive circuits of an ultrasound system. One way is to utilize a monolithic sheet or patterned flexible circuits. For transducer architectures utilizing monolithic sheet or patterned flexible circuits, the circuits are generally constructed from metal foils, typically copper foils, that are rolled, and annealed or electro-deposited on a drum, peeled off and either bonded to a polymer film carrier or have a polymer film directly applied to a surface. These foils are typically available in various thicknesses ranging from about 0.6 mils to about 5.25 mils (3/8 ounce/ft.sup.2 to 1 ounce/ft.sup.2 weight foils). The thickness of the foils may be increased by about 0.04 mils to 0.1 mils with the addition of a diffusion barrier layer, usually nickel, and a corrosion resistant layer, usually gold. The thinnest commonly available metallic material for flexible circuits utilize 0.5 ounce/ft.sup.2 foil having a thickness ranging from about 0.7 to about 0.9 mils. These metal foils have a conductive layer of uniform thickness and the interconnect circuits are fabricated by the subtractive etching a circuit design in the uniform conductive layer that extends across the surface of a polymer film. The presence of thick metal, i.e., about 1/2 ounce weight, in the acoustic path limits the performance of the device due to mass-loading. U.S. Pat. No. 4,404,489 (Larsen et al.) describes such a prior art flex circuit and method of fabrication.
Since the metal layer has about the same acoustic impedance as the transducer piezoelement formed of piezoelectric material, typically lead zirconate titanate (PZT), the thickness of the transducer piezoelement must be sacrificed for foil thickness to achieve a particular operational frequency. This results in a thinner layer of piezoelectric material which is more difficult to fabricate and handle. In addition, to maintain efficient piezoelement performance, certain width to thickness ratios need to be employed. As the piezoelectric material gets thinner the piezoelement width must be proportionally narrowed. This lowers the frequency ceiling for a producible device with respect to the limits of piezoelement definition, i.e., dicing and material strength of the piezoelement structure. In addition, the prior art methods are limited to a single layer of copper of uniform thickness in the trace areas as well as the active acoustic path and interconnect areas. These limitations are imposed by the requirements for low resistance signal pathways and the associated inability to remove a limited and uniform amount of material in the acoustic pathway.
It is thus desirable, especially for high frequency transducers, to minimize the metal thickness in the flex circuit making contact with the piezoelectric material. This requirement will be referred to as a low mass connection in the acoustic region.
In methods where transducer piezoelements are disposed on or laminated to a monolithic or patterned sheet using adhesives it is difficult to obtain sufficient bond strength between the foil and the PZT that will survive the fabrication process, particularly piezoelement definition. Mechanical roughening, such as abrasion of the bonding surface with an emery cloth or sand paper can improve the adhesive bond, but may compromise the integrity of the electrical path. Foil treatments, such as electrodeposition of metal nodules on the surface of the foil also improve adhesive bond strength, but may increase the effective bond line thickness between the foil and the transducer piezoelements.
It is thus desirable to improve the adhesive bonding quality of the transducer element to the flex circuit without compromising the electrical path or roughening the surfaces to the extent that increased thickness of the epoxy bond lines result.
In transducer architectures where a low mass connection is desired several approaches have been used. One method involves the hand soldering of wires or traces directly to the electrode of the transducer piezoelements. Another method, the Tape Automated Bonding (TAB) method, commonly used in the wafer industry, can be applied to transducer piezoelement connections. In the TAB method, a TAB jumper is soldered or welded directly to the transducer piezoelement or an intermediate connector using automated tooling. Still another method, ultrasonic or thermocompression wire bonding can be used to attach a low mass lead directly to the transducer piezoelement and then to an intermediate connector.
In methods involving soldering or bonding traces directly to the transducer piezoelements, the connections are generally made one at a time thus making assembly of the transducer complicated, cumbersome, and time consuming. In addition, the connection point of the trace to the piezoelement has a very small area which does not provide a redundant path for the transducer piezoelement electrode. Thus, if the coupling of the trace to the piezoelement is compromised or if there is a discontinuity in the PZT electrode outside of the connection point area, the piezoelement becomes electrically isolated either totally or partially and may be rendered unusable. Moreover, in direct termination methods where mass and gang termination is an available option, such as with TAB, significant heat and/or mechanical damage can result from the termination process. Also, a separate provision must be made for connecting the transducer piezoelements to ground. Typically, such provisions involve soldering a foil or applying a conductive epoxy bead to the ground electrode of the PZT.
Another disadvantage of these methods is that they may not permit the use of a prefabricated acoustic backer during the construction process of the transducer array because the wirebonds or leads are on the backside of the acoustic path and thus the acoustic backer must be applied in a liquid phase after the bonds have been made.
It is thus desirable to provide a flexible circuit that has low mass in the acoustic region so as to minimize deleterious mass loading effect on the acoustic performance and assist in maintaining structural integrity of the transducer piezoelement through the fabrication processes. It is also desirable to provide a flexible circuit that maintains electrical continuity across the entire piezoelement electrode area, providing redundancy to the PZT electrode and having low resistance over the entire flex circuit trace length. It is also desirable to provide a simple connection method to allow easy mass termination during the fabrication process. It is also desirable to provide a similar connection method for the ground path. It is also desirable to effectively minimize electrical crosstalk between adjacent signal traces. Finally, it is desirable to provide a flexible circuit having an improved adhesive bond strength between the flexible circuit and a bonding adhesive. Furthermore, it is desirable to provide a flexible circuit that acts as a heat sink to carry heat away from the piezoelements.